Sum values from one column vector based on another column vector

How do I compute this without using any loops or conditionals?

I want to sum the values in column B based on if the value in column A is a 1

So for example I want to add 34 and 28 only and save it to a new variable

A B

1 34

2 78

4 3

1 28

Answer 1

[DGM]

Assuming A and B are different variable names with the same length:

A = [1; 2; 4; 1];
B = [34; 78; 3; 28];
mysum = sum(B(A==1),'all')

mysum = 62

Assuming they're both columns in the same array:

A = [1 34; 2 78; 4 3; 1 28];
mysum = sum(A(A(:,1)==1,2),'all')

mysum = 62

[Allison Sims]

That worked thank you very much!